1. Field of the Invention
The present invention relates to material coating and, more particularly, to a method and apparatus for coating a substrate with a deposition material in a vacuum.
2. Brief Description of the Prior Art
Coating a substrate with a deposition material typically involves vaporizing the deposition material in a vacuum such that the vaporized deposition material condenses onto a substrate that is at a lower temperature than the temperature of the vaporized deposition material.
In the production of organic-based devices, a is thin, flat, film-like substrate is coated with a chemical coating, usually organic based, on at least one side of the substrate. The substrate material may be glass or a plastic/polymeric material and though typically planar in configuration, may also consist of a curved or non-planar surface. The size of the substrate being coated is generally limited to a few square inches due to technical capability limitations of current material sources.
During fabrication of most organic-based devices, such as organic-based LED displays, organic-based lasers, organic-based photo-voltaic panels, and organic-based integrated circuits, chemicals or deposition materials are typically applied to the substrate in a vacuum, using a point source crucible A, shown in FIG. 1, or a modified point source crucible. When the chemicals are heated, the chemicals vaporize and radiate away from the point source crucible A, through an exit aperture B, in a generally cosine-shaped emission plume C. A substrate D is then typically held in a fixed position or rotated within the emission plume C with a planar side E of the substrate D facing the point source crucible A. A certain amount of vaporized chemicals deposits on the planar side E of the substrate D, forming a film coating.
In some applications, modified point sources are used to produce a gaussian (non-uniform).flux distribution. Examples of modified point sources include R. D. Mathis-type boats, Knudsen cells, or induction furnace sources. A general drawback of point or modified point source crucibles, however, is their design. First, the ability to control evaporation rates of chemicals involves sensitive, precise control over material temperatures and temperature gradients with low heat capacities and poor thermal conductivity. Point sources/gaussian material sources typically use radiant reflectors, insulation, and baffling to create good evaporation rates for metals and salts at higher temperatures of 1,000-2,000xc2x0 C. However, these material sources are inappropriate for evaporating organic-based chemicals at lower temperatures of 100-600xc2x0 C. Excessive heat applied to many organic-based chemicals will spit the chemicals out of the material sources, destroying any film being grown on the substrate and requiring the vacuum system to be taken out of service in order to be cleaned and reloaded. Another problem is that the vaporized chemicals frequently condense into the exit apertures of the crucibles of point or modified point sources. The condensation of the vaporized chemicals begins to alter or occlude the exit aperture, causing chemicals to fall back into the crucible""s heated interior, and spit onto the substrate. This spitting ruins the homogenous distribution of the chemical film, because films having spit defects exhibit higher surface roughness values and may exhibit pinhole defects entirely through the deposited layers. The source aperture condensation also degrades the uniformity of the deposited film by altering the flux emission distribution.
Another disadvantage of both point and modified point source crucibles is that no axis of flux uniformity can be found. Point source and modified point source crucibles produce relatively uniform films only when flux angles are kept small. As shown in FIG. 2, flux angles xcex1, xcex2, and xe2x8ax96 are measured from a normal axis N extending from the exit aperture of the point source crucible to lines L1, L2, and L3 representing the edge of the cosine-shaped plume C shown in FIG. 1. The only way to keep the flux angle small, such as the angle xcex1 shown in FIG. 2, is to greatly increase the separation distance, or throw distance, between the point source crucible A and the planar side E of a substrate, such as those substrates referred to by reference numerals D1, D2, and D3. For example, substrate D2 would need to be moved to the position of substrate D3 to be fully coated, while keeping the flux angle xcex1 constant. Such a move would increase the throw distance from TD2 to TD3. Similarly, if substrate D3 is moved to the position of substrate D1, i.e., from TD3 to TD1, then only a small portion of substrate D3 would be coated, and the deposited coating would be much less uniform. Film uniformity is a very important characteristic of organic layers utilized for photonic and electronic applications as the fabricated devices will not operate properly, if at all, if the organic-based films are not maintained at a 95 percent or higher level of uniformity.
Throw distances can be predicted in order to achieve a uniform film of 95 percent or higher. If this uniformity requirement is applied to a 6-inch square substrate, for example, then a throw distance of approximately 2 xc2xd feet may be required. By comparison, a 24-inch square substrate would require a throw distance of 9 xc2xd feet. This increasing throw distance destroys the ability to develop a productive process, because the rate of film growth is inversely proportional to the square of the distance between the crucible and the substrate.
Film growth rates of organic-based materials are typically expressed in single Angstroms per second. For example, a throw distance of one foot or less would be desirable for coating a 12-inch substrate with a 95 percent uniform film coating 1000 Angstroms thick. At the one-foot throw distance, a typical chemical deposition rate would be 18 Angstroms per second, which equates to a coating time of approximately fifty-five seconds conversely, at a throw distance of 9 xc2xd feet, the typical deposition rate is 2 Angstroms per second, resulting in a 1 xc2xd-hour deposition time.
In addition to increasing film growth rates, increases in throw distance significantly increase production costs. First, vacuum chambers must be large enough to accommodate the increased throw distances, requiring larger vacuum deposition chambers as well as more powerful vacuum pumps. Second, there is a substantial waste of expensive chemicals, since an increase in throw distance decreases deposition efficiency. Third, because the vaporized organic material that does not reach the substrate is deposited on an interior wall of the vacuum chamber, the vacuum chamber must be removed from productive service and cleaned more frequently. Cleaning is expensive because some chemicals, such as those used to produce organic liquid electronic displays, are toxic as well as expensive. Costs are further exaggerated because point or modified point source crucibles only hold between 1 and 10 cubic centimeters of chemicals. Therefore, only a few substrates can be coated before the vacuum chamber must be brought to atmosphere, the vacuum chamber cleaned, the crucibles refilled, and the vacuum chamber re-evacuated.
It is therefore an object of the present invention to produce a method and apparatus for coating a substrate in a vacuum that allows larger substrates to be coated without increasing throw distances as the width of a substrate increases, allowing more deposition material to be deposited on the substrate during coating, reducing loading downtime, and reducing cleaning time.
In order to help solve the problems associated with the prior art, the present invention generally includes a vacuum deposition system for coating a substrate with a deposition material. The vacuum deposition system includes a vacuum chamber and a material source positioned inside the vacuum chamber. The material source has a body which extends along a longitudinal axis, a substantial longitudinal emission component, and defines an interior cavity and an exit aperture fluidly connected to the interior cavity. A heat source is positioned adjacent to the body of the material source.
A substrate to be coated, having a width measured parallel to the longitudinal axis of the body, may be positioned inside the vacuum chamber, wherein a throw distance, measured between one side of the substrate and the exit aperture, remains constant as the width of the substrate increases. Preferably, the substantial longitudinal component of the body of the material source is equal to the width of the substrate or less than the width of the substrate.
A deposition material is loaded into the interior cavity of the body of the material source. The deposition material is selected from the group including an organic-based chemical and an organic-based compound. The deposition material is heated by the heat source and emitted through the exit aperture along the substantial longitudinal emission component of the body of the material source.
The material source may have a body in the shape of an open trough having two longitudinally extending sidewalls and a pair of endwalls, wherein the longitudinally extending sidewalls and the endwalls define the interior cavity of the body. The body of the material source may further define an upper end positioned adjacent to the exit aperture and a base, with the heat source being a heating coil having a greater number of heating elements positioned at the upper end of the body than at the base of the body. The exit aperture may extend continuously along the substantial longitudinal emission component of the body and ribs positioned in the internal cavity defined by the body of the material source.
The material source may also have a first conduit defining an internal cavity and a first exit aperture fluidly connected to the internal cavity, wherein the body is a second conduit received in the internal cavity of a first conduit. The first exit aperture defined by the first conduit maybe aligned with the exit aperture defined by the second conduit or, the first exit aperture defined by the first conduit may be aligned in a non-coincident configuration with the exit aperture defined by the second conduit. Regardless of body type, a process control apparatus may be connected to the body of the material source.
One method of coating a substrate using a material source and a vacuum chamber includes the steps of:
a. positioning the material source in the vacuum chamber, the material source having a body which extends along a longitudinal axis, has a substantial longitudinal emission component, and defines an interior cavity and an exit aperture fluidly connected to the interior cavity;
b. positioning a substrate in the vacuum chamber, opposite the exit aperture defined by the body of the material source;
c. loading a deposition material in the interior cavity defined by the body of the material source;
d. evacuating the vacuum chamber to create a vacuum;
e. heating the deposition material in the internal cavity of the body of the material source;
f. emitting vaporized deposition material along the substantially longitudinal component of the body; and
g. moving the substrate through the vaporized deposition material.
The substrate may be moved through the vaporized deposition material at a constant velocity. When substrate coating is complete, the substrates can move to another process or the vacuum chamber can be opened, the coated substrates removed, new substrates added, the vacuum chamber re-evacuated, and the above process steps repeated.
One type of material source for use in vacuum deposition of a deposition material onto a surface of a substrate includes two bodies, such as a point source crucible, a modified point source crucible, or a combination, with each of the two bodies defining an interior cavity and at least one exit aperture fluidly connected to the interior cavity and a heating element positioned adjacent to each of the two bodies, wherein the two bodies are aligned along a common longitudinal axis to form a substantial longitudinal emission component. A process control apparatus may be connected to one of the two bodies of the material source, and the interior cavities of the two bodies are configured to receive deposition material selected from the group including an organic-based chemical and an organic-based chemical compound.
Another type of material source for use in vacuum deposition of a deposition material onto a surface of a substrate includes a body which extends along a longitudinal axis, has a substantial longitudinal emission component, an defines an interior cavity and at least one exit aperture fluidly connected to the interior cavity and a heat source positioned adjacent to the body of the material source. The exit aperture may extend continuously along the substantial longitudinal emission component of the body and ribs may be positioned in the internal cavity defined by the body of the material source. The material source may have a body in the shape of an open trough having two longitudinally extending sidewalls and a pair of endwalls, wherein the longitudinally extending sidewalls and the endwalls define the interior cavity of the body.
The material source may also include a first conduit defining an internal cavity and a first exit aperture fluidly connected to the internal cavity, wherein the body is a second conduit received in the internal cavity of a first conduit. The heat source is positioned adjacent to the first conduit or the second conduit, the heat source including a first layer of heat conductive electrical insulation, a second layer of conductive material, and a third layer of heat conductive electrical insulation. The first exit aperture defined by the first conduit may be aligned with the exit aperture defined by the second conduit or the first exit aperture defined by the first conduit may be aligned in a non-coincident configuration with the exit aperture defined by the second conduit.
These and other advantages of the present invention will be clarified in the Detailed Description of the Preferred Embodiments taken together with the attached drawings in which like reference numerals represent like elements throughout.